Most laser drivers have circuitry to detect faults and a single line to indicate the presence of a fault. For instance, Freitag et al., U.S. Pat. No. 5,999,549, disclose an apparatus for detecting laser fault conditions and generating a laser fault control signal, which is used to disable the laser under prescribed conditions. However, the apparatus has only a single input coming from a monitor diode, and produces a single output to indicate a fault condition.
In another example, Romm, U.S. Pat. No. 6,792,020 discloses a driver circuit for an opto-electrical transmitter with a first fault activation circuit in the driver which activates a fault signal that disables the driver, and a second fault activation signal, which times the duration of any periods during which a digital parameter of the driver is outside a target range. From the fault signals provided it is not possible to distinguish which fault condition is the source, nor can the separate fault signals recorded or stored in the driver circuit.
There are several sources of faults that may not be mutually exclusive. The originating fault may cause other faults to be detected as well, for example the original fault may cause the laser to be disabled, which may result in another fault to be reported. In previous designs, fault signals were recorded without any indication of the source or the order in which they occurred. When a fault occurred, there was no way to determine its original source. Determining the source of the fault may thus require a lengthy debugging process.
An object of the present invention is to improve the fault sensing system to provide more information for the identification of originating fault.
Another object of the present invention is to determine the order in which fault signals occur.